function* fibonacci() {
  let [prev, curr] = [0, 1];
  while (true) {
    yield curr;
    [prev, curr] = [curr, prev + curr];
  }
}

function* take(iterable, limit) {
  for (const item of iterable) {
    if (limit <= 0) return;
    yield item;
    limit--;
  }
}

const fibs = [...take(fibonacci(), 5)];
fibs.push(fibs[0] + fibs[1]);
console.log(fibs);